1. Field of the Invention
The present invention relates to a television signal receiver capable of receiving color television signals of different systems, e.g., the NTSC (National Television Systems Committee) system, the PAL (Phase Alteration Line) system, and the MAC (Multiplexed Analogue Component) system which is a time division system.
2. Description of the Prior Art
Satellite television broadcasting is finding wider use in recent years. Some television signal receivers, which have begun to be commercially available, are capable of receiving high-definition television signals broadcast by satellite and also VHF and UHF signals of the NTSC and PAL systems. For example, certain color television signal receivers that are commercially available in Japan can receive NTSC television signals, BS television signals, and high-definition television signals. Some European color television signal receivers are able to receive PAL and MAC television signals.
According to the PAL system, the color subcarrier is modulated by a balanced modulator with two chrominance signals, and one of the chrominance signals is inverted in phase per scanning line in the process of generating the same carrier chrominance signal as that used in the NTSC system. There have been proposed various versions of the MAC system, and they are known as the A-MAC system, B-MAC system, C-MAC system, D-MAC system, and D2-MAC system. FIGS. 1A, 1B, and 1C of the accompanying drawings show the basic signal time-division multiplex formats of the B-MAC, C-MAC, and D2-M systems, respectively, which are presently in use or planned for use. The common concept among these MAC variations is that analogue-comonent video signals are broadcast on air according to the MAC system, and differs from the NTSC system in which luminance and chrominance signals are frequency-multiplexed and broadcast on air. According to the MAC system, the luminance signal is time-divided for all lines, the color difference signal is time-divided alternately per line, and the audio signal is converted into a digital signal for transmission.
The basic signal time-division multiplex formats of the B-MAC, C-MAC, and D2-MAC systems shown respectively in FIGS. 1A, 1B, and 1C will be described below. In each of these systems, the color difference signal in one horizontal scanning time H (64 ms) is time-compressed to 3 : 1, and the luminance signal in one H is time-compressed to 3 : 2. In the B-M system, the bandwidth after time compression is 7.5 MHz. In the C-MAC system and the D2-MAC system, the bandwidth after time compression is 8.4 MHz.
In the B-MAC system shown in FIG. 1A, the digital audio signal and independent data are subject to binary or binary/quaternary conversion, time-divided in the baseband during the retrace interval of the video signal, and frequency modulated together with the video signal. In the C-MAC system shown in FIG. 1B, the digital audio signal and independent data are time-compressed so that they can be time-division-multiplexed during the retrace interval of the video signal, and then modulated by phase-shift keying (PSK), and the retrace interval of the video signal is replaced with the PSK-modulated audio signal. The digital signal to be multiplexed is transmitted as a packet. The D2-MAC system shown in FIG. 1C operates as follows: When C-MAC television signals are to be transmitted by CATV (cable television), since the signals are wideband signals, they cannot be transmitted over the conventional cables. Therefore, the video signal is limited in band, and the digital signal to be multiplexed is reduced to half. Then, the video signal is modulated by VSB-AM, and the digital signal is time-division-multiplexed in the retrace interval of the video signal.
A television signal receiver which is capable of receiving PAL and MAC television signals determines whether the received signal is a PAL or MAC television signal each time a channel is selected. More specifically, the television signal receiver has a MAC signal processing circuit for reading a flag "1" which indicates the MAC system, from received MAC-system digital data, so that the received television signal is of the MAC system. However, it takes about 1 second for a PLL circuit in a decision circuit in the MAC signal processing circuit to stabilize, and it also takes about 1 second to select a desired station or channel after the received television signal has been determined as being of the MCA system. Therefore, the channel selection process is relatively time-consuming since about 2 seconds are consumed each time a desired channel is selected.